With the development of industries all over the world, the use of petroleum and a natural gas has increased, and thus concern for the development of minor marginal oil fields or deepwater oil fields, which have been ignored because of low economical efficiency, has been rising.
Drillships equipped with drilling facilities for developing such oil fields have been developed in various forms, along with the development of seabed mining techniques.
In oil drilling methods used on the sea, in the past, rig ships or fixed platforms for only seabed drilling were mainly used for seabed drilling while being at anchor at one point on the sea. In recent years, drillships equipped with high-tech drilling equipment and manufactured in the same form as general ships so as to be able to be propelled by their own engines have been developed and used for seabed drilling.
As for structures of the above-mentioned drillships, a moon pool vertically passes through the center of a hull to move down a drilling pipe to a seabed. In such a moon pool, during navigation of the drillship, a very strong turbulent flow may be generated. The turbulent flow is generated by an interaction between a vortex separated at an end of the moon pool which is located on a bow side and a free surface. The turbulent flow makes the free surface unstable, and increases a time variation in resistance, thereby degrading the resistance performance of the drillship.
FIG. 1 shows a side view for describing a shape of a conventional general moon pool installed in a drillship. An external flow S10 introduced into a moon pool 50 of the hull of the drillship moves from a bow side 10 to a stern side 11. In this case, a vortex suppression block 3 protrudes from the bottom of the moon pool 50 which is located on the bow side toward the stern side in order to suppress a vortex V10 generated by the external flow (main flow). The flow is separated at an edge 10a of the vortex suppression block 3, and the vortex V10 is generated due to instability of a shear layer.
Due to the flow, the vortex bumps against an edge 11a of the moon pool 50 which is located on the stern side, and thereby a high-pressure turbulent flow is generated. As such, a time variation in resistance applied to the moon pool 50 increases. In addition, there is a problem in that a part S11 of the flow is introduced into the moon pool 50 by the vortex colliding with the edge located on the stern side, thereby disturbing a free surface W10 in the moon pool 50.
The related art for reducing the above-mentioned turbulent flow is shown in FIG. 2. FIG. 2 is a side cross-sectional view showing a shape of the moon pool having a flow stabilizing apparatus installed on a drillship. As shown in FIG. 2, an inclined protrusion 20 protrudes from the edge of the moon pool 50 which is located on the bow side, and is inclined toward the seabed. The inclined protrusion 20 is intended to control a vortex generated at an end of the inclined protrusion, and adjust a separation angle θ of an external flow S20 passing through the moon pool 50.
However, in the turbulent flow reducing structure described above, there are problems as follows. The inclined protrusion 20 that is additionally formed on the vortex suppression block 3 for suppressing the vortex at the edge of the bow side protrudes from the bottom of the hull toward the seabed. Thus, the inclined protrusion 20 may be restricted by depth of the seawater when the drillship is built. Further, the inclined protrusion 20 becomes too large when attempting performance improvement while keeping the separation angle constant. In addition, the separation angle increases, and thus an average value of resistances applied to the drillship increases when attempting performance improvement while keeping a height of the inclined protrusion 20 constant.
In addition, the related art for suppressing the flow in the moon pool is disclosed in Korean Patent Application No. 10-2007-37729 (entitled Anti-Sloshing Device in Moon Pool). This flow suppressing apparatus installed in the moon pool distributes and absorbs flow energy of the seawater generated inside the moon pool, suppresses a sloshing phenomenon and an overflowing phenomenon generated inside the moon pool, and delays a vortex generated inside the moon pool. Thereby, the speed of a ship can be improved. A major configuration thereof has a plurality of moon pool plates that are vertically installed at a bow side, a stern side, and both sides of the ship that correspond to inner walls of the moon pool; and a bottom block of the moon pool which is attached to a lower end side of the bow side of the inner wall of the moon pool in a central direction of the moon pool, and formed so as to maintain a level equivalent to a bottom surface of the ship. It is characterized that the moon pool plates and the bottom block of the moon pool have a protruding width that does not interfere with a maximum working area, a plurality of ends of an upper side among the moon pool plates are installed lower than a free surface of the seawater when the ship is located at a working location, and a plurality of ends of a lower side among the moon pool plates are installed lower than the free surface of the seawater when the ship is sailing.
Such a conventional technique has an advantage in that the apparatus reduces the flow introduced into the inside of the moon pool, and thereby the free surface in the moon pool can be stabilized because the apparatus distributes and absorbs the flow energy after it has been introduced into the moon pool.
However, there are disadvantages in that there is a reduced ability to control the flow and the vortex separated from the edge of the bow and thereby reduce pressure variability caused by collision of the streamlines and the vortex at the edge of the stern, because introduction of the external flow into the moon pool is not reduced. Additionally, installation of moon pool plates with an excessively complex configuration creates a problem in that weight and cost are increased.
In addition, still another preceding art for suppressing a flow in a moon pool is disclosed in Korean Patent Application No. 10-2008-87278 (entitled Moon Pool Having Anti-Flow Device). The moon pool having an anti-flow device separates a flow of the seawater to form a boundary layer for the purpose of a reduction in flow velocity or blocking of the running water introduced through a moon pool, and thereby performing a direct resisting function and an anti-flow function through friction, interference, and bubble generation among the flows of the running water. It is characterized that the seawater flow self-suppressing apparatus is installed in a singular or multistage structure, in which the seawater flow self-suppressing apparatus includes a seawater flow change structure that is installed at a vertical wall near a waterline of the moon pool, and is made of angled steel so as to change a flow direction of the seawater to be horizontally disposed along the vertical wall, and a running water flow separation structure that is formed in a funnel shape so as to separate the flow of the running water from a lower portion of the seawater flow change structure to be horizontally disposed along the vertical wall, and thereby forming the seawater boundary layer.
Such related art has an advantage in that the device separates the flow of the seawater to form the boundary layer for the purpose of the reduction of the flow velocity or the blocking of the running water introduced through the moon pool, and thereby a self-blocking ability of the seawater anti-flow device can primarily be used, and a friction force by the separated flow can be used.
However, there are disadvantages in that there is a reduced ability to control the flow and the vortex separated from the edge of the bow and thus reduce pressure variability caused by collision of the streamlines and the vortex at the edge of the stern, because introduction of the external flow into the moon pool is not reduced, and a problem in that an increase in weight and an increase in manufacturing cost of the drillship occur due to installation of the seawater flow change structure that is horizontally disposed along the vertical wall with an excessively complex configuration.
In addition, as techniques for suppressing a flow in a moon pool of a drillship, there are a plurality of preceding arts for reducing the flow in the moon pool such as Korean Utility Model Registration Application No. 20-2006-24344 (floating restraint structure in the moon-pool of ship) and Korean Patent Application No. 10-2007-124862 (flowing reduction structure of moon-pool structure of drill ship), but most of them relate to an additional apparatus configuration inside the moon pool. Most of them have a disadvantage in that there is a reduced ability to control the flow and the vortex separated from the edge of the bow and thus reduce the pressure variability caused by the collision of the streamlines and the vortex at the edge of the stern, because introduction of the external flow into the moon pool is not reduced, and a problem in that an increase in weight and an increase in manufacturing cost of the drillship occur due to addition of an additional complex apparatus configuration in the moon pool.